The present invention relates to particulate suspending agents for use in high-temperature and/or at near-neutral or higher pH (e.g., greater than about 5) wellbore applications, and methods relating thereto.
Subterranean operations often involve fluids that include particulates. For example, cement slurries include hydraulic cement particulates, and drilling fluids often include particulate weighting agents. To maintain particulates in suspension, the fluids are often viscosified. As the bottom hole circulating temperature of a wellbore increases, the viscosity of a fluid decreases, which is known as thermal thinning, which can cause undesirable settling of the particulates from the fluid.
Particulate settling can cause the density of the fluid to change, which can lead to, inter alia, an influx of formation fluids. To inhibit settling, suspending agents, e.g., crosslinked polymers, can be added to fluids. As the fluid temperature increases, the suspending agent is thought to increase the viscosity of the fluid, for example, by breaking crosslinks to release a polymer into the fluid. One important feature of a suspending agent is that it not adversely affect low-temperature rheology.
Existing suspending agents, e.g., guar or guar derivatives crosslinked with borate, delay crosslink breakage sufficiently to allow mixing and pumping of a fluid without imparting an excessively-high viscosity. However, existing suspending agents are known to degrade above 300° F. This temperature limitation makes these suspending agents more impractical for use in higher temperature applications. Synthetic polymers crosslinked with esters or amides are effective at temperatures in excess of 400° F., but the crosslinks may not be degraded rapidly enough at near-neutral pH values (e.g., about pH 5 to about pH 8) to offset thermal thinning as the fluid heats up in the wellbore.